Because of increase in engine powers and severer environmental regulations of exhaust gases, piston rings having ion-plated, hard chromium nitride coatings for high scuffing resistance and wear resistance have long been used. Because piston rings used in severe environment in engines should have long lives, hard coatings are required to be as thick as 10-60 μm. Because Cr has a relatively high vapor pressure among metals, a chromium nitride coating can be formed to a required thickness relatively easily, so that it has been conveniently used in the piston ring industry.
Because such chromium nitride is generally hard but is easily broken, various measures such as crystal orientation control, structure control, porosity control, the addition of third elements, etc., have been conducted. However, there is recently demand to further improve the thermal conductivity of CrN, because recent trend of higher compression ratios and higher loads in engine specifications causes new problems of higher combustion chamber temperatures and knocking. In addition, when pistons are made of aluminum alloys (hereinafter called simply “aluminum”), softened aluminum causes the wearing of ring grooves and is adhered to piston rings. To tackle this problem, heat is required to be dissipated from pistons to cooled cylinder walls through piston rings, by effectively utilizing the thermal conduction of piston rings. However, CrN with low thermal conductivity hinders the thermal conduction of piston rings. Non-Patent Reference 1 reports that the thermal conductivity of CrN is 0.0261-0.0307 cal/cm·sec·deg (corresponding to 10.9-12.9 W/m·K (by SI), and Non-Patent Reference 2 reports that the thermal conductivity (room temperature) of a thin CrN film of about 3 μm is about 2 W/m·K (when measured by a light pulse thermoreflectance method. Though the thermal conductivity is measured in planar and thickness directions, it is difficult to measure the thermal conductivity of a coating of several tens of μm in a thickness direction. Apart from such difficulty, it is considerably lower than the thermal conductivity of 20-30 W/m·K of SUS440B and SUS420J2, typical steels for piston rings, and such low thermal conductivity is considered a large factor of hindering the thermal conduction.
Titanium nitride (TiN) has also been proposed for hard coatings for piston rings, and actually used in some of piston rings. The thermal conductivity of TiN is 0.07 cal/cm·sec·deg (room temperature) (corresponding to 29.3 W/m·K (by SI) in Non-Patent Reference 1, and 11.9 W/m·K (in Non-Patent Reference 2, 3-6 times as high as that of CrN. However, it has extremely high residual compression stress inside the coating, which suffers cracking, breakage, peeling, etc. when it is thick. Accordingly, TiN cannot actually be coated to a thickness required for piston rings. Patent Reference 1 describes that the control of TiN to have a columnar crystal structure provides a TiN coating with smaller residual stress, enabling the TiN coating as thick as 80 μm at maximum, and that having a predominant orientation in a (111) or (200) plane in parallel to the coating surface is particularly preferable from the aspect of scuffing resistance.
Patent Reference 2 describes that mere increase in the intensity ratio of a (111) plane in a TiN film may not provide sufficient wear resistance, and that excellent wear resistance is obtained by increasing the intensity ratio of a (111) plane and reducing the intensity ratio of a (220) plane in X-ray diffraction.
Though the orientation of a (111) plane, a surface of a close-packed structure of TiN, in parallel to the coating surface is effective for improving the scuffing resistance and wear resistance as described in Patent References 1 and 2, TiN coatings with too strong orientation actually have large residual stress as described above, difficult to be used for piston rings. For example, even if a TiN coating as thick as up to 30 μm were formed, the peeling of the coating, etc. would occur when actually used for piston rings.